Great are the works of the Lord: They are studied by all who delight in them. (Psalm 111:2)
 

Volcanism & Extrusive Rocks

May 18, 1980 eruption of Mt. St. Helens

Molten rock/plastic -- magma

Magma -- when it reaches the surface before it solidifies -- is called lava

Volcanic activity or volcanism may involve lava or not -- gas, solid rock, ash etc.

PYROCLASTIC DEBRIS or TEPHRA - Rock fragments produced by volcanic explosion
 

Magma could solidify below the ground surface or above the surface. If the molten material becomes solid on the earth's surface, it is called an EXTRUSIVE igneous rock.

A land form created by volcanism is a volcano; a hill or mountain formed by extrusion of lava or ejection of rock fragments. Volcanism helps the geologists to understand or provide clues about what happens below the ground surface (magma, gases, rocks from eruptions) from which chemical conditions as well as the temperature and the pressure within the earth's crust are known.

Volcanic Features

Vent -- opening through which an eruption takes place

Crater -- basin-like depression over a vent at the summit of the cone

Caldera -- similar to crater but larger >1 km

Flank eruption -- lava pouring out from a side vent
 

Three Major Types of Volcanoes (Shield, Cinder, Cone & Composite)

Shield Volcanoes -- these are broad, gently sloping cones constructed of solidified lava flows -- low costly flow is factor, covers wider area

Generally (like in Hawaii) the lava is basaltic, meaning it is low in silicon and high in iron, magnesium and calcium oxides.
 

Basalt flows (two types) --

PAHOEHOE (pay-hoy-hoy) -- characterized by its ropy surface appearance
AA (ah-ah) -- pasty mass appearance -- half solidified but the front is moving along to give it a pasty massive look

Spatter cone -- when a solidified lava develops a steep side cone which may be due to escape of entrapped gas

Cinder cone -- a volcano constructed of loose rock fragments ejected from a central vent slope (are steep ~ 30 -- smaller & rarely exceed 500m)

Fragments making up the cones are pyroclastic

(Pyroclasts or tephras can be of different sizes: dust, ash, cinders ~ 4-32mm, bombs & blocks -- large pyroclasts )

Local accumulation of gas in a magma cause eruptions that build cinder cones

Composite (Stratovolcano) - constructed of alternating layers of pyroclastics and rock solidified from lava flows. Slopes are intermediate but the cinder cone and shield volcanoes have steeper slopes. Composite volcanoes are built up over a long span of time. Nearly larger and better known composite volcanoes occur within the Circum-Pacific belt ring of fire and the Mediterranean belt.

Volcanic Domes -- steep sided, dome or spine-shaped mass of volcanic rock formed from viscous lava that solidified in or immediately above a volcanic vent (Mt. Pelee, Mt St. Helens)

Glowing avalanche -- very hot flow of pyroclastics; when hardened -- called welded tuff
 

Lava Floods

All extrusive rocks are not associated with volcanoes. Plateau Basalts -- (Columbia's Plateau, Dacca Plateau) produced by vast outpours of lava during the geologic past.

Sometimes these lava form in vertical columns -- mostly 6-sided-columnal structures in jointly submarine eruptions (oceanic ridges, pillow lava video).
 

Sources of Lava

Plate tectonics

1. Basalts (oceanic crust)

2. Geologically young andesite volcanoes are aligned along Circum-Pacific and Mediterranean belts

3. Most contemporary submarine volcanism occurs along the mid-oceanic ridges

Plate tectonics and the oversized basalt sea floor spreading

Hot spot

Global pattern of volcanism

. Ocean-ridge

. Convergence zone

. Ocean-ocean

. Ocean-continent

. Inter-plate
 

Effects of Volcanism on Humans/Life

1. Existence of Hawaii - land for habitation

2. Geothermal Energy - Italy, Mexico, New Zealand, USA (California), Argentina

3. Catastrophes - Mt. St. Helens; Pompeii - Mt. Vesuvius

Chemistry of Volcanic Rocks

Slica (SiO₂) is the most abundant component of most volcanic rocks, varying from 45-75% of the total weight of volcanic rocks. This variation accounts for the differences in appearances in the rocks.

Mafic Rocks -- usually dark in color. Basalt is the most common mafic rock. Rocks with 50% silica content (by weight) are considered silica-poor (oxides of Al (Al₂O₂), Calcium (Ca), Magnesium (Mg), Iron (FeO and Fe₂ O₃).

Felsic Rocks -- Silica-rich rocks (65% or more) which tend to have smaller amounts of the above listed oxides (Ca, Ma, Fe) but 25-35% of Na (Na₂O), Al (Al₂O₂) and K (K₂). Name comes from feldspar; Rhyolite is the most abundant volcanic rock with a felsic (light colored) composition.

Intermediate Rocks -- Andesite -- usually medium-dark gray color. The chemical composition of the magna determines which minerals and how much of each will settle to form Igneous rocks.
 

Identification of Extrusive Rocks

Based on composition and texture.

Composition

Silica amount determines viscosity, violence of eruptions and also the particular rock that is formed. Using the minerals, one can identify the rocks but usually most extrusive rocks are fine grained (and use of microscope would be helpful).

Dark/Light
 
 

Vol of Rock	RHYOLITE	ANDESITE	BASALT
100%			Ferromagnesium
75%		Plagioclase
50%	K-feldspar	Plagioclase
25%	(Orthoclase)
0	Quartz

75% SiO₂ 45% SiO₂

Increase < Na₂O/K₂O

CaO, FeO, MgO

Felsic-- Intermediate-- Mafic
 

Rhyolite - usually cream-colored, tan or pink.

Basalt - olivine or augite - dark green or black.

Andesite - gray to green.
 

Textures: Grain size - Rocks most important textured characteristic.

(Phaneritic (coarse), Aphanitic (fine grained) and Pyroclastic or Fragmental. Other types of textures are Glassy, vesicular, Pegmatitic, Porphyritic (porphyry, phenocrysts))

Obsidian (chemistry is similar to Rhyolite) and Pumice are classified based on texture alone while all others are by composition and texture.
 

Fine-grained: (Mineral grain < 1 mm). Size is determined by rate of cooling and viscosity.
 

Porphyritic Textures: (Large crystals in a fine matrix). Phenocrysts - large crystals.
 

Fragmental Texture: Tuff, Volcanic Breccia. Textures due to trapped gases. Pumice, Vesicle-cavity caused by gas
 
 

Intrusive Activity and the Origins of Igneous Rocks (Plutonism)
 

Intrusive Rocks - Rocks that crystalize from magma in-situ ( in the ground).
 

Gabbro - low - A mafic, coarse-grained igneous rock is the intrusive equivalent of basalt (bulk of extrusive rocks.
 

Granite - bulk of intrusive rocks - is the equivalent of Rhyolite - less. Why?
 

How do you tell Intrusive from Extrusive igneous rocks?

1. Mineralogy/Texture

2. Laboratory experiments

3. Presence of Country Rock - forcing of intrusive igneous rock into the country rock.

4. Presence of Inclusions

5. Baked or metamorphism along contact zone

6. Chill zones at the edges of the intrusive rocks
 

Intrusive Bodies - Intrusions or Intrusive bodies - size, shape and relationship to surrounding rocks are important structure of the earth's crust.
 

Naming and Classification of intrusive igneous are based on:

1. Size

2. Shape

3. Depth of fixture

4. Geometric relationship of Intrusive Igneous rock to the country rock.
 

Shallow Intrusive Structures-

Size of grain are fine e.g. Andesite (can be intrusive and extrusive)

1. Volcanic Neck e.g. Ship Rock in New Mexico

2. Dikes and Sills Tubular and discordant - Dike Sill Concordant

3. Laccolith mound (or mushroom) type

4. Lopolith (spoon shaped)
 

Deep Intrusive Structures (Bodies)- A more refined drawing

Igneous Rocks formed at several kilometers are called Plutonic Rocks

1. A pluton (like Dike and Silly but are irregular and larger

2. Stock - a small discordant pluton with exposed area less than 100 square kilometers.

3. Batholith - if area greater than 100 square kilometers.
 
 
 
 
 
 
 
 
 
 
 
 
 

Identification of Intrusive Rocks
 

Coarse Grained	Granite	Diorite	Gabbro	Utramafic
Fine Grained	Rhyolite	Andesite	Basalt	Peridotite
100
75
50
25
0	Quartz

75% SiO₂ 45% SiO₂

K₂O & Na₂O

Ca, FeO, MgO

1 Felsic intermediate Mafic Ultra Mafic
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Common Minerals of Igneous Rocks
 

	Minerals	Chemical Composition	Silicate structure
Mafic	Olivine	(Mg, Fe)2 SiO4	Isolated Texture
	Pyroxene	Mg? With Ca, Al	Singe Chain
	Amphibole	Mg, Fe Ca, Na (Si8O22(OH)2	Double Chain
	Biotite	K, Mn, Fe, Al, Si3O18 (OH)2	Sheets
Felsic	Muscovite	KAl3Si3O10(OH)	Sheets
	Plagioclase	NaAlSi3O8CaAl2Si2O8	Framework
	K-Feldspar	KAl2Si2O8	Framework
	Quartz	SiO2

 
 

Coarse grained Rocks -

Pegmatites - very coarse grains that are larger than 1 centimeter.

Some rocks are named solely on their textures alone:

Obsidian - glass

Pumice - Frothy volcanic glass

Volcanic Breccias - Coarsely fragmented volcanic rock

Tuff - volcanic rock composed of fine fragments.

Prophyritic and Vesicular - terms used to describe Igneous Rocks

Xenolith

Phenocrysts
 

Ultramafic Rocks - No fine grained equivalent. No Quartz, feldspar - entirely ferromagnesia minerals. Temp 2000 C is very high for melting, few are found?
 

Varieties of Granite - classification and problems
 
 
 

How Magma Forms

Melting - Sources of heat for melting:

1. Geothermal gradient ~ 2.5 C/100 m (25 C/km)

2. Hot mantle plumes - (Hot spots) e.g Yellowstone National Park

3. Friction in ex building areas

4. Heat transfer - occurs from magma and form other structure?

5. Radioactivity - Uranium
 

(Rising temperature, Lowering of Pressure, Addition of water, spreading centers mantle plumes and subduction zones)
 

Factors that control melting temperatures

1. Pressure - melting point generally increases with increasing pressure

2. Water under pressure - lowers melting point

3. Effect of mixed minerals

 Types of Magma & how they evolved

Magma types: felsic, intermediate and, mafic
 

Differentiation and Bowen's Reaction Theory

Different ingredients (minerals) separating from original homogenous mixture.

Bowen Reaction Series - Sequence in which minerals crystalize from a cooling magma.

Fractional crystallization

Continuous and Non continuous branches of crystallization
 

Magma TypeDiscontinuous BranchContinuous Branch

BasalticOlivine - isolated silica tetrahedron Ca-Rich Plagioclase

(low silica) Augite) Pyroxene single chain Ca-Na rich plagioclase

Andensitic(hornblende) Amphibole Double chain Na rich plagioclase

Mica Biotite sheets

Muscovite (if water pressure is high)

GraniticOrthoclase

(high silica) Quartz
 

If pressure is high

Muscovite k feldspar Quartz 3-D structure ~ 600C orthodox (K)

Experiments gave rise to Magmatic infferentiation illustrate

mafic wall

olivine

Pyroxene

cristobalite

SiO₂Pyroxene + SiO₂
 

Problem with magmatic differentiation - does not account for all the observed Rocks.

Crystal setting and magma formations

Assimilation - intermediate magma

Mixing of magmas

Partial melting
 

Stopping - fracturing country rock above the magma chamber. Basaltic magma is more fluid i.e. less viscous, so travels rapidly & easily two fissures to the surface rather than solidifying in the rock.
 

Igneous Activity and Plate Tectonics

Basalt and Ultramafic Rock - most are found associated with mid-oceanic ridge
 

Subduction zone - andesitic
 

Granite | Andensitic - Convergence between 2 plates

Partial melting of basalt

Melting of Sed. Rock

Partial melting of mantle

Assimilation of control rocks

Partial melting of lower crust

Heating by transient magma and by under platting?

Combination of processes